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The natural world today has been changed pretty radically by humans. Stand in the center of New York City or at the edge of a giant corn field and the human signature is obvious.

But when you think about the past, it's easy to imagine a world where the landscape was pristine and untouched. Where people lived in perfect harmony with their surroundings, and the ecosystem was just the backdrop where they lived their lives. But, in many cases, that's not how it was.

Many of the existing ecosystems people think of as pristine — things like jungles, forests, and savannahs — were, in fact, shaped by the humans of the past. Granted, these people weren't changing the planet on a global scale like we are now. But by understanding the ways that past societies did affect their ecosystems, we can uncover histories that have been forgotten, lost, or in some cases purposefully buried.

Now, throughout the natural world, there are countless examples of ecosystems obviously shaped by humans from the past. There are ecosystems shaped by irrigation, crop cultivation, or the introduction of new species — to name a few. But it's not always obvious that you're looking at a disturbed environment, because nature takes a lot of different shapes and forms.

Like, ever since they were colonized by the British, the rainforests of the Solomon Islands east of Papua New Guinea have often been thought of as a pristine ecosystem. And flying over them, you might see an unbroken blanket of trees. But after hearing oral histories about people who lived there pre-colonization, researchers looked a little closer and spotted sections of the woods where the tree species weren't distributed like you'd expect in an untouched rainforest.

There seemed to be patches where the forest filled in a former clearing. These patches were, essentially, abandoned settlements. In general, strange distributions of species like this one can be a strong clue that we're not looking at pristine nature.

The signs may be subtle, but for those familiar with the life cycle and history of plants and animals, an out-of-place grove of trees might stand out like a rhino in the middle of Times Square. In fact, there's a similar type of clue right here in the U. S.

Honey locust trees naturally grow in rocky and dry upland areas of the eastern United States, but they can also sometimes be found near rivers and floodplains throughout the region, which might seem unusual. But today, scientists think these lowland populations may actually descend from trees planted by the Cherokee, who grew them for food and medicine. Overall, odd distributions of species may be signs that people were clearing forests for wood or grazing, encouraging the growth of useful plants or clearing out harmful ones, or even introducing whole new species.

But there are other telltale signs of altered ecosystems, too. For instance, aerial or satellite photography can reveal unnatural features of the environment that are impossible to see from down below, like human-made mounds, lines, or other feats of engineering. And a close-up look can reveal subtler clues, like changes in the soil.

Things like unusual levels of nitrogen, silt, or charcoal might be signs of past humans fertilizing the soil, irrigating crops, or setting fires. Finally, there's another kind of evidence. History.

This can include records like government documents or photographs, but it can also include oral histories. Traditionally, scientists have been wary of using oral histories as evidence. Since, by nature, they're so subjective, they're often seen as unreliable.

But oral histories often contain specialized traditional knowledge and can offer insight on how people interacted with their environment. This means they can be both a really useful starting point for finding human-altered environments as well as a lens for interpreting them — especially in places with suppressed or hidden histories. Overall, each of these types of information can help us figure out how seemingly pristine ecosystems might actually be the echoes of things like agricultural practices or woodland management.

And by understanding how past societies interacted with their environments, we can build a more nuanced picture of what their lives might have been like. For example, let's go to a Bolivian region called the Llanos de Moxos. Here, in the southwest Amazon basin, the rainforest opens up into a large, flat, grassy savanna.

It's threaded by multiple rivers, so the savanna often floods. But the landscape is dotted with little forested hills that are just tall enough to avoid the yearly floods, letting the trees grow tall and lush. These are called forest islands, and many are roosting sites for bird species, like the endangered blue-throated macaw.

Decades ago, it was thought that this area was essentially untouched and unable to sustain a large human population. But since then, a number of discoveries have changed how we view the region. The scientific debate around the level of human influence here and throughout the Amazon region isn't settled — but scientists have realized that the floodplain contains remnants of significant human activity, including works of engineering like canals, causeways, and raised fields.

In fact, the forest islands themselves may largely be the product of human engineering. Some may be natural, but at least some of these forest islands seem to actually be the remnants of ancient levees constructed sometime between 1,500 and 1,000 years ago. And to draw this conclusion, scientists used just about every tool in the toolbox.

For instance, they used mapping techniques, including satellite mapping, to create diagrams of these earthworks. Some of the mounds were simple, but others were arranged into complex, intentionally-created shapes or connected by causeways. Meanwhile, through archaeological excavations, they found pottery, shells, and human remains in the mounds.

They even checked out old plant material and found evidence of domesticated gourds, maize, and the root vegetable manioc. This suggests that not only was this area populated, but the people there may have already been domesticating plants — even though full-blown agricultural societies probably didn't show up until much later. Scientists are still arguing over these results and what they could mean.

But at least some of the evidence suggests that, far from being a pristine ecosystem, the unique environment at Llanos de Moxos is actually an echo of the agricultural specialization that happened there more than a millennium ago. And even here in the U. S., clues in our ecosystems can tell us something about the past.

For example, take the forests along the Eastern United States. Accounts from Europeans described these forests as open, kind of like giant, wooded parks, with very little undergrowth. And some people have assumed that this was how the forests naturally grew.

That, if only people would stop messing with them, the landscape would return to that stable, natural state. Except, that open, park-like structure is not how those forests naturally grow. A natural forest would have a whole tangle of smaller saplings and underbrush growing up in the shadow of those larger trees.

The only reason they wouldn't look that way is if frequent, small fires were burning through the region. In that case, the underbrush would regularly burn away, while large trees could survive. And even today, you don't see as many fire-sensitive trees, like maples, growing in certain woods.

Instead, they're dominated by tougher oaks and pines, which supports the idea that these regions experienced frequent fires. The thing is, regular, controlled burns like that aren't the work of something natural, like lightning. They're the work of people.

Aside from these environmental clues, several lines of evidence, including historical records, archaeological evidence, and soil samples point to the fact that this environment was shaped by human hands. The groups living on these lands — many of which are still around today — had and still have detailed traditional knowledge about how fire affects the ecosystem. And setting small, frequent fires could have had a number of benefits.

For instance, it could have made hunting easier, decreased the risk of giant, catastrophic fires, fertilized the soil, and encouraged the growth of useful plant species. In fact, we now recognize that people all throughout North America used fire to shape their environments. So, environments around the world hold clues about the people who shaped them, even long in the past.

And there's a lot to learn by investigating them. For one, research like this can confirm oral histories, lending weight to narratives that may have been ignored. And in some cases, like on the Solomon Islands or in North America, this evidence can help us better understand the true population size pre-colonization.

It can also help us rediscover or better understand how pre-colonial populations shaped their environments in a way that supported them. But perhaps most importantly, it can help us put together pieces of history that are missing from the usual historical narratives. In some cases, we've forgotten things simply because time and human memory will naturally bury certain pieces of history.

But, sometimes the reason we don't recognize ecosystems as affected by human hands is because much of history has been written by colonizers. We sometimes think of science as being separate from humanity and all its messiness, but situations like this are reminders that that isn't true. Many times, colonizers have misunderstood, lied about, or purposefully set out to stop indigenous practices and destroy indigenous culture, knowledge, and histories.

Plus, any observations that were made in good faith during colonization were often made during times of extreme population collapse and social disruption. So, in places that have been colonized, we often can't rely on the prevailing history that's been passed down to paint an accurate picture of how indigenous people lived before colonial times. We mention this not to give a history lecture, but to acknowledge that this does affect huge amounts of science — especially because, even today, indigenous people have often been ignored or barred from participating in and shaping science and history.

But at times, the land itself has held on to pieces of history that have been lost or rewritten. As a whole, these studies tell us that wherever people have gone, we have affected and shaped the world around us — whether that's in our homes or our ecosystems. This is true today and has been true for a long, long time, even if it's not always obvious.

But by studying the human echoes in ecosystems around the world, we can better understand our pasts and how our decisions today will shape the future. Thank you for watching this episode of SciShow! And thank you to all our patrons, whose support and curiosity about the world makes these videos possible.

We couldn't do it without you! If you'd like to learn more about how to support SciShow, you can do that over at patreon.com/SciShow. [♪ OUTRO].